1. Field of the Invention
This invention relates to a sheet material punching device, more particularly to a sheet material punching device used in a finisher that performs post-treatments to a sheet of paper transported from an image formation apparatus.
2. Background Art
For example, the sheet material punching device disclosed in the Patent Document 1 has a plurality of punches provided in the longitudinal direction of a frame and die holes formed correspondingly to the plurality of punches, wherein the rotational motion of a driving mechanism is converted by a cam mechanism into reciprocating motions of the plurality of punches in a punching direction. The plurality of punches and the die holes jointly form an array of holes in a sheet material.
The sheet material punching device disclosed in the Patent Document 1 includes: two slide arms allowed to reciprocate along the longitudinal direction of the frame, the two slide arms making the plurality of punches reciprocate in the punching direction along with their own reciprocating motion by the intermediary of a plurality of links; cams provided in the respective slide arms, the cams having cam grooves capable of converting the rotational motion of the driving mechanism into the reciprocating motions of the slide arms; and cam followers provided, for example, in a drive gear, to be engaged with the cam grooves, wherein an array of holes to be formed by the plurality of punches is changed by switching the reciprocating motion of one of the slide arms to the reciprocating motion of the other.
In the sheet material punching device of this type, as illustrated in, for example, FIG. 20A, there is an interval T between a cam follower 202a provided in a drive gear 202 and a cam groove 201a provided in a slide arm 201 which is one of slide arms, a slide arm 201, is at an initial position. The interval T is formed in a predefined dimension so that a large operating resistance is not generated during an initial drive of a driving source, for example. Because of the interval, the slide arm 201 does not start to move forward in the reciprocating motion immediately after the drive gear 202 starts to rotate clockwise on the drawing.
After the slide arm 201 is reciprocated; moved forward (FIGS. 20B and 20C) and then moved backward (FIG. 20D), the cam follower 202a further slightly rotates from an initial position illustrated in FIG. 20A toward an inverted position through 180° (FIG. 20E) clockwise on the drawing. While the cam follower 202a is moving from the position illustrated in FIG. 20D toward the inverted position illustrated in FIG. 20E, the slide arm 201 does not follow the movement of the cam follower 202a. Therefore, the slide arm 201 fails to return to the initial position illustrated in FIG. 20A. This consequently shortens the reciprocating distance of the slide arm 201, causing unfavorable events. For example, the holes may not be formed in the sheet, or the punches may fail to punch through the sheet.
To avoid these problems, the sheet material punching device disclosed in the Patent Document 1 is provided with a tension spring 204, which is a biasing member, between a frame 203 and the slide arm 201 as illustrated in FIGS. 20F to 20H. The tension spring 204 constantly keeps biasing the slide arm 201 in a direction where the slide arm 201 moves back to the initial position. As a result, the slide arm 201 that ended the backward movement of the reciprocating motion (FIG. 20F) can still follow the movement of the cam follower 202a and accordingly return to the initial position (FIG. 20G).